1. Field of the Invention
The present invention relates to a method and apparatus for controlling a position control apparatus. More specifically, the present invention relates to a method and apparatus for controlling calibration of the origin position in such a position control apparatus where the position of an object being controlled is controlled in an incremental manner.
2. Description of the Prior Art
In automatic welding apparatuses, automatic painting apparatuses, other types of industrial robots, and the like, positional information concerning a point being controllably moved is programmed in terms of the coordinates for the purpose of moving a welding torch, a printing gun or spray, manipulator, and the like. Thus, the torch, or the like, is position controlled to a positional point determinable by the positional point information, for example, by means of a point to point system, simply referred to as a "PTP" system.
A so-called incremental system can be employed as a servo system for position control in such automatic welding apparatuses. As is well known, an incremental system may be defined as a system wherein each present positional point of an object being controlled is represented in terms of an incremental amount with respect to the coordinates of the previous point assumed by the object immediately before each said present positional point, and is characterized by simplicity in structure. Nevertheless, in the case where position control is effected in an incremental system, a disadvantage is encountered in that, if the apparatus is deenergized by turning off of a power supply (for example, the numerical value stored in a feedback buffer or counter for representing the present positional point of the object being controlled is volatilized and disappears), the numerical value in the buffer is not regained when the apparatus is restarted. Therefore, in the case of restart of the apparatus, the object being controlled must be brought to the origin position of the coordinates, whereupon the positional control is restarted, as is well known to those skilled in the art.
Conventionally, for the purpose of calibration of the origin position, the object being controlled had to be either directly and manaully, or through manual operation of a position control apparatus, brought to the origin position of the coordinates. However, the former is tiresome in operation, while the latter is disadvantageous in that it is difficult to stop the object being controlled correctly at the origin position. An improved system for calibration of the origin position was proposed by the same inventors, and was set forth in Japanese Patent Laid Open Application No. 111585/1976. The referenced patent laid open application appearing in the laid open gazette discloses an automatic position control apparatus wherein an origin returning means is provided for moving an object being controlled in the direction toward the origin coordinates, and the object being controlled is made to pass the origin by manually operating the origin returning means; a feedback buffer is cleared when an overrun position is reached, and the object being controlled is adapted to be automatically positioned in an appropriate position as commanded thereafter. Even with the above described improved system set forth in the referenced patent laid open gasette, a problem remains to be solved in that an open circuit servo system for manual operation is required for the origin returning means for allowing overrun of the object being controlled.
In order to solve the above described problem, and thus to eliminate the necessity of a particular servo system, one might think of loading a relatively large appropriate numerical value in the above described position feedback buffer or counter on the occasion of once again turning on the power supply, whereupon the object being controlled is moved in the direction toward the origin. However, this necessitates a register of large capacity, and accordingly makes the cost larger. Nevertheless, assuming that the same is achieved by the use of a number of digits required for ordinary position control, then other various problems are encountered in that it is impossible to allow for overrun, or the output from the encoder is obtained even after the origin position is exceeded, so as to cause run away of the object being controlled, and so on.